1. Field of the Invention
The present invention relates to a signal distinction circuit for detecting a discrimination signal having a predetermined frequency, and more particularly, to a signal distinction circuit suitable for detecting a discrimination signal having a low frequency included in an FMX stereophonic broadcasting. The present invention also relates to a signal distinction circuit for detecting a signal by a counter is not malfunctioned when the signal is instantaneously cut off.
2. Description of the Prior Art
Recently, an FMX stereophonic broadcasting is proposed to improve the S/N of an FM stereophonic broadcasting and enlarge a service area. According to the FMX stereophonic broadcasting, a compressed stereophonic difference signal is transmitted simultaneously with the signal of the FM stereophonic broadcasting. The amplitude of the compressed stereophonic difference signal is modulated by a subcarrier of 38 KHz in an orthogonal relationship therewith. A discrimination signal of 10 Hz is included in the signal of the FMX stereophonic broadcasting so that the FMX stereophonic broadcasting is distinguished from the FM stereophonic broadcasting. Therefore, in order to receive the FMX stereophonic broadcasting, it is necessary for a receiver to detect the discrimination signal and switch the receiving mode thereof to the FMX stereophonic receiving mode.
The FMX stereophonic broadcasting is described in detail on pages from 11 through 15 of the September, 1986 edition of the magazine "JAS JOURNAL".
The discrimination signal of 10 Hz can be detected by a signal distinction circuit as shown in FIG. 9. Referring to FIG. 9, a stereophonic pilot signal of 19 KHz applied to a first input terminal 1 is divided into approximately 74 Hz by a first dividing circuit 2 and then, applied to a counter 3 as a clock signal. A discrimination signal of 10 Hz applied to a second input terminal 4 is supplied to a pulse generating circuit 6 through a bandpass filter 5. Therefore, a pulse signal having the same frequency as that of the discrimination signal applied to the input terminal 4 is produced from the pulse generating circuit 6. The pulse signal thus produced is divided by a second dividing circuit 7 and then, supplied to the counter 3 as an actuating signal. As a result, the counter 3 counts the clock signal while the pulse signal maintains an "H" level. A detecting circuit 8 detects whether or not the value counted by the counter 3 equals to a predetermined value.
The signal discrimination circuit shown in FIG. 9 is provided with a level detecting circuit 9 to detect the level of a signal outputted from the output terminal of the bandpass filter 5. The level detecting circuit 9 produces an "L" level signal when the level of the signal supplied thereto is above a predetermined value and produces an "H" level signal when the level of the signal is below the predetermined value. Therefore, when the level of the signal falls for some reason, the level outputted from the level detecting circuit 9 becomes the "H" level, which causes the detecting circuit 8 to be reset.
However, the method of distinguishing the presence of the discrimination signal by the counter has a disadvantage that when the discrimination signal is not applied to the distinction circuit, the counter stops counting the clock signal. As a result, the signal distinction circuit maintains the condition in which the discrimination signal has been applied. For example, when a signal is not received by a receiver caused by a sudden retuning or when a receiver mounted on a vehicle is not capable of receiving a signal because of the fact that it is traveling in a tunnel, the receivers maintain the condition in which they have received discrimination signals despite that discrimination signals are not applied thereto, which causes, for example, an indicator to light erroneously.
Further, when a discrimination signal of 10 Hz included in the signal of an FMX stereophonic broadcasting is distinguished by the distinction circuit shown in FIG. 9, a bandpass filter 5 having a center frequency of 10 Hz is used. It is to be noted that the normal lower cut-off frequency of a program signal of the FMX stereophonic broadcasting is 50 Hz and the normal level of the program signal is greater than that of the discrimination signal. Therefore, a portion of the program signal is applied to the pulse generating circuit 6 through the bandpass filter 5. The pulse generating circuit 6 produces a pulse by detecting a zero crossing. Therefore, when a program signal having a low frequency and a large amplitude leaks from the bandpass filter 5, a narrow width-pulse is generated at the output terminal of the pulse generating circuit 6, which may cause the counter 3 to malfunction in response to the narrow width-pulse.
In addition, in receiving a radio broadcasting having a discrimination signal, the discrimination signal is distorted and its level falls by a multipath interference or an external noise. It is desirable that the condition of a receiver does not change even though the level of the discrimination signal falls so that a listener enjoys listening to the radio. However, in the receiver provided with the signal distinction circuit shown in FIG. 9, the level of the level detecting circuit 9 outputs an "H" level signal caused by the instantaneous fall of the level of the discrimination signal instantaneously, which causes the detecting circuit 8 to be reset. As a result, the state of the receiver changes. For example, when a receiver is receiving the FMX stereophonic broadcasting, the receiver is switched to receive the FM stereophonic broadcasting by an instantaneous fall of a discrimination signal of 10 Hz and a noise level fluctuates, so that a listener feels uncomfortable.
Furthermore, when a multipath interference occurs when the receiver is receiving the FMX stereophonic broadcasting, a received signal and a discrimination signal are distorted. As a result, the value counted by the counter 3 constituting the signal distinction circuit is out of a predetermined range of the detecting circuit 8. As a result, an "L" level signal is outputted from the detecting circuit 8 and the receiving mode is switched to the FM receiving mode. When the signal received by the receiver is subjected to a frequent interference such as by the multipath interference, the level of the signal outputted from the detecting circuit 8 are frequently switched, which causes the indicator to flash and the receiver to an inappropriate state that the FMX stereophonic broadcasting is received in the FM mode.